Active fan blade noise cancellation system

Abstract

An active noise cancellation device and method for reducing fan blade noise over a broad spatial area. The device comprises a microphone, a band pass filter, and audio amplifier, and a speaker array. The microphone captures the sound produced by the fan and converts it into an electrical signal. This electrical signal is input to the band pass filter which has a center frequency equal to product of fan speed and the number of fan blades which comprise the fan. The band pass filter attenuates any signals other than the primary harmonic sound produced by the rotating fan blades. The filtered signal from the band pass filter is input to the audio amplifier. The audio amplifier conditions the signal for input to the speaker array. Accordingly, each speaker in the array receives and outputs an audio signal of equal amplitude and phase as that produced by the rotating fan blades. The speaker array is mounted on or close to the fan hub in a symmetric pattern commensurate with the fan blade geometry. In general, the number of speakers which comprise the array corresponds to the number of fan blades. The position of the speaker array is adjusted until a maximum destructive interference occurs between the sound produced by the rotating fan blades and the speaker array. With the addition of a phase shifter, the active noise cancellation device can be utilized to provide noise cancellation for variable speed fans.

What is claimed is: 
     
       1. A system for reducing fan blade noise, said system comprising: (a) sensor means for sensing the sound waves produced by rotating fan blades and converting said sound waves into an electrical signal stream, said sensor means being positioned in proximity to the blades of said fan;   (b) filter means adapted to receive said electrical signal stream from said sensor means as input, said filter means attenuating extraneous signals from said electrical signal stream and outputting a filtered signal stream;   (c) amplifier means connected to said filter means for adjusting the amplitude of said filtered signal stream to coincide with the amplitude of the sound waves produced by said rotating fan blades; and   (d) a speaker array connected to said amplifier means for converting an output signal stream from said amplifier means into audio signals, said speaker array being positioned in proximity to said fan blades such that said audio signals are out of phase with said sound waves, thereby resulting in destructive interference, said speaker array comprises at least a number of individual speaker elements equal to the number of fan blades comprising said fan, wherein said individual speaker elements are arranged in a symmetrical pattern and each outputs said audio signals at the identical amplitude and phase thereby producing a spatial pressure field identical to the spatial pressure field created by said rotating fan blades.   
     
     
       2. The system for reducing fan blade noise according to claim 1, further comprising a phase shifting means for adjusting the phase of said output signal stream from said amplifier means to compensate for variable fan speeds. 
     
     
       3. The system for reducing fan blade noise according to claim 2, wherein said sensor means is a microphone, said microphone being positioned in proximity to the outer edge of said fan blades. 
     
     
       4. The system for reducing fan blade noise according to claim 3, wherein said microphone comprises a windscreen to prevent damage to said microphone and to reduce wind noise. 
     
     
       5. The system for reducing fan blade noise according to claim 4, wherein said filter means is a band pass filter having a center frequency equal to the product of the fan speed and the number of fan blades which comprise said fan. 
     
     
       6. The system for reducing fan blade noise according to claim 5, wherein said amplifier means is an audio amplifier. 
     
     
       7. The systems for reducing fan blade noise according to claim 1, wherein the diameter of said speaker array is less than or equal to a hub of said fan. 
     
     
       8. The system for reducing fan blade noise according to claim 1, wherein the diameter of said speaker array is greater than the diameter of rotation of said fan blades. 
     
     
       9. An active noise cancellation system for reducing fan blade noise over a wide spatial area, said system comprising: (a) sensor means for sensing the sound waves produced by rotating fan blades and converting said sound waves into an electrical signal stream, said sensor means being positioned in proximity to the blades of said fan;   (b) filter means adapted to receive said electrical signal stream from said sensor means as input, said filter means attenuating extraneous signals from said electrical signal stream and outputting a filtered signal stream;   (c) amplifier means connected to said filter means for ajdusting the amplitude of said filtered signal stream to coincide with the amplitude of the sound waves produced by said rotating fan blades; and   (d) a speaker array connected to said amplifier means for converting an output signal stream from said amplifier means into audio signals, said speaker array being positioned in proximity to said fan blades such that said audio signals are out of phase with said sound waves, thereby resulting in destructive interference; and   (e) phase shifting means for adjusting the phase of said output signal stream form said amplifier means to compensate for variable fan speeds.   
     
     
       10. The active noise cancellation system for reducing fan blade noise according to claim 9, wherein said sensor means is a microphone, said microphone being positioned in proximity to the outer edge of said fan blades. 
     
     
       11. The active noise cancellation system for reducing fan blade noise according to claim 10, wherein said microphone comprises a windscreen to prevent damage to said microphone and to reduce wind noise. 
     
     
       12. The active noise cancellation system for reducing fan blade noise according to claim 11, wherein said filter means is a band pass filter having a center frequency equal to the product of the fan speed and the number of fan blades which comprise said fan. 
     
     
       13. The active noise cancellation system for reducing fan blade noise according to claim 12, wherein said amplifier means is an audio amplifier. 
     
     
       14. The active noise cancellation system for reducing fan blade noise according to claim 13, wherein said speaker array comprises at least a number of individual speaker elements equal to the number of fan blades comprising said fan. 
     
     
       15. The active noise cancellation system for reducing fan blade noise according to claim 14, wherein said individual speaker elements are arranged in a symmetrical pattern and each output said audio signals at the identical amplitude and phase thereby producing a spatial pressure field identical to the spatial pressure field created by said rotating fan blades. 
     
     
       16. The active noise cancellation system for reducing fan blade noise according to claim 15, wherein the diameter of said speaker array is less than or equal to a hub of said fan. 
     
     
       17. The active noise cancellation system for reducing fan blade noise according to claim 15, wherein the diameter of said speaker array is greater than the diameter of rotation of said fan blades. 
     
     
       18. An active noise cancellation method for reducing fan blade noise over a wide spatial area, said method comprising the steps of: (a) sensing the sound waves produced by said fan blades as they rotate and converting them into a signal stream;   (b) filtering said first signal stream to attenuate extraneous signals from said signal stream;   (c) adjusting the amplitude of said signal stream to coincide with the amplitude of the sound waves produced by said rotating fan blades;   (d) generating an audio signal stream from said signal stream in proximity to said fan blades; and   (e) adjusting the phase of said audio signal stream until maximum destructive interference results between said audio signal stream and said sound waves.   
     
     
       19. The active noise cancellation method according to claim 18, wherein said step of filtering comprises passing said signal stream through a band pass filter having a center frequency equal to the product of the fan speed and the number of fan blades which comprise said fan. 
     
     
       20. The active noise cancellation method according to claim 19, wherein said step of generating an audio signal stream comprises utilizing a symmetrical speaker array having at least a number of individual speaker elements equal to the number of fan blades to generate a spatial pressure field equal to that generated by said rotating fan blades. 
     
     
       21. The active noise cancellation method according to claim 20, wherein said step of adjusting the phase comprises positioning said symmetrical speaker array such that said spatial pressure field is one hundred eighty degrees out of phase with that produced by said rotating fan blades.